Though I don't think the sun is the main culprit in skin cancer it sure the hell isn't the SPF..
Certain heritages have the prediposition to skin cancer,particularly red headed light skinned people of Irish and Scottish descent. Mediterranian people and Africans tolerate the sun much better.
I personally believe that our diet,low in protective anitoxidants and good nutrtition overall hampers our ability to handle over exposure to the sun.
I live in Arizona and I get alot of intense sun through hiking and just mowing my lawn. I have my self checked by a doctor often.